Once you have removed the thermostat inspect the condition of the main body check for any cracks or broken pieces and check the valve to make sure it is closed. If the valve is open the thermostat has failed and needs to be replaced. To check the operation of the thermostat prepare a pot of water on the stove deep enough to cover the thermostat completely. Place the thermostat in the pot of water ( it should be suspended to avoid heat from stove to be directly transferred ) , turn the stove on a medium/high flame, the thermostat should open right before the water comes to a boil. If the water has boiled and the thermostat valve is still closed the thermostat has failed and needs to be replaced.